1. Field of the Invention
This invention relates generally to semiconductor processing, and more particularly to apparatus for laser beam induced phenomena inspection of integrated circuits and to methods of using the same.
2. Description of the Related Art
Fault-isolation techniques are critical to the development and manufacture of large-scale integrated circuits such as microprocessors. Examples of such techniques include those methodologies which exploit thermal beam induced phenomena such as Thermally Induced Voltage Alteration (“TIVA”), Seebeck Effect Imaging (“SEI”) and Infrared Laser Optical Beam Induced Resistance Change (“IR-OBIRCH”). These techniques are often effective in localizing electrical opens and shorts that can be significant manufacturing concerns.
TIVA and SEI utilize a scanning infrared laser to isolate a defect within a die by thermally altering the defect's electrical characteristics. The basic setup uses a constant current source for biasing the device under inspection (“DUT”) and detects the change in the voltage demand due to the localized heating by the laser. This technique requires the use of time constant delay to amplify the change in voltage for effective detection. However, integrated circuits, such as microprocessors, require high current biasing that is often beyond the specification limits of commercially available current sources. In addition, biasing in the constant current mode may not turn on all the operation modes of certain mixed signal integrated circuits.
Like TIVA and SEI techniques, IR-OBIRCH maps the infrared laser position to changes in the I-V characteristics of the device under inspection which result from localized laser heating of a defect. In this case, a constant voltage source is implemented for biasing and the resultant change in current is monitored Although IR-OBIRCH avoids the technical issues associated with using a constant current source, the sensitivity may be less than that provided by TIVA. Moreover, the detection of current change is difficult to implement in general.
Another drawback associated with conventional thermal beam inspection techniques is the potential for a discontinuity to exist between the length of time that the incident laser beam strikes a given pixel and the response time of the signal detection circuitry. Most conventional laser scanning microscopes scan the laser beam continuously across a frame. Each pixel is momentarily illuminated for a time period that is fixed by the frame size, e.g., 512×512 and the scan rate, e.g., 5.0 seconds/frame. If the illumination time per pixel is less than the response time of the signal detection circuitry, then the sensed signal will tend to smear into adjacent pixels and produce a characteristic tail pattern that can obscure the behavior of adjacent structures.
The present invention is directed to overcoming or reducing the effects of one or more of the foregoing disadvantages.